1. Field of the Invention
This invention relates to the simulation of living tissue monitored by a pulse oximeter. In particular it relates to simulation of arterial and non-arterial blood oxygen characteristics as may be measured by a pulse oximeter.
2. Description of the Related Art
Pulse oximeters to date have been designed to detect the oxygenation of arterial blood by using the differential absorption of hemoglobin by red and infra-red light as a mechanism for determining blood oxygen. Specifically, the oximeter alternately illuminates tissue (for example, here, one side of a finger) with alternating flashes of red and infra-red light of constant amplitude and at a repetition rate that is high compared to the maximum heartbeat rate. As the heart takes a beat the finger expands and contracts slightly, increasing the path length, and thereby the attenuation of the light applied to the finger. The attenuated light is captured by a photodetector, and the now amplitude modulated light flashes are processed to determine blood oxygen by detecting the red and infra-red heartbeat created modulation waves and processing them.
Simulators, used to evaluate oximeter performance, have therefore essentially been attenuators and amplitude modulators, taking the red and infra-red light flashes and/or their electrical analogues, and attenuating and amplitude modulating them as would living tissue.
There is, however, a problem with the practical use of oximetry as described above. If the patient moves then the motion could also change the path length through the monitored tissue, creating a modulation based not on arterial pressure related to expansion/contraction alone but also on expansion/contraction of tissue containing non-arterial blood. The oxygenation level of non-arterial blood tissues is at a lower value than that of the arterial blood tissues. Also, the amplitude of modulation caused by motion can be much higher than the arterial value. The result is that the oximeter can lock onto the motion signal and misread the blood oxygen value. This is a serious problem when, for example, a patient leaves the operating room and oximetry is disturbed by shivering. In other words, the oximeter may fail just as its information is needed most.
However, new oximeters have arrived on the market which appear to be able to discriminate between arterial and non-arterial blood oxygenation. These will require a new type of simulator for testing; one which can simultaneously simulate arterial and non-arterial blood characteristics; pulse shape, amplitude, and oxygenation level.
These new oximeters take advantage of the fact that in an unmoving hand (or other body part upon which oximetry is practiced), the pulsatile spectrum of non-arterial blood is virtually DC, but in a motion situation, a non-arterial "pulse" is created by virtue of the motion of the body compressing and decompressing the non-arterial tissues and changing the optical path length from oximeter probe emitter to sensor. The result of motion therefore is a non-arterial "pulse" in addition to the cardiac generated arterial pulse. The above definition of "non-arterial pulse" is used throughout the specification. The two pulses (arterial and non-arterial) naturally have differing frequency spectra. The differing spectra provide an opportunity for:
a. Separation of the two pulses using modern signal processing techniques, despite the fact that their detection is simultaneous by the oximeter; PA1 b. Using the red/infra-red ratio of each of the separated pulses to obtain an oxygenation value for each; and PA1 c. Choosing the higher oxygen value as the arterial. PA1 a. The use of current sensing resistors; PA1 b. The use of current transformers; PA1 c. The use of opto couplers. PA1 a. Directly applying the modulated pulses across an impedance to create a current according to I=E/Z; PA1 b. Driving a current through a transformer; PA1 c. Driving current through an opto coupler. PA1 1. The red and infra-red LED drive analogs. These are obtained from the oximeter. PA1 2. Red and infra-red attenuation factors. These are determined within the simulator. They establish the simulated bulk attenuation of tissue. PA1 3. Red and infra-red arterial plethysmographic modulation waveforms. These are determined within the simulator. The ratios and absolute amplitudes of these waveforms determine oxygen value and signal strength, and their fundamental frequency determines simulated heartbeats/minute. PA1 1. Preserving the start/stop timing of the LED drive inputs in the output pulse timing. PA1 2. Outputting red and infra-red pulses whose unmodulated amplitude is set by the red and infra-red attenuation factors. This sets the so-called "DC" values of the pulses, as shown in FIG. 2. PA1 3. Outputting red and infra-red pulses which are amplitude modulated by the red and infra-red plethysmographic waves, respectively, or other simulation waves, as desired. This sets the so-called "AC" values of the pulses, as shown in FIG. 2. PA1 a. Create superimposed arterial and non-arterial waveforms; PA1 b. Give each of those waveforms an independent simulated oxygenation value; PA1 c. Give each of those waveforms an independently set amplitude; and PA1 d. Give each of those waveforms an independently set fundamental frequency and/or waveshape. PA1 a. Entirely mathematically by computer--that is, the dual modulation may be done in conjunction with the computer reading (by a/d conversion) the "Y" input, and the double modulated result presented directly at point "X" (by d/a conversion); PA1 b. By computer controlled hardware in various configurations--that is, the dual modulation can also be accomplished with the mathematical operations appropriate to modulation being partly done in the computer and partly in the hardware. PA1 a. For the non-arterial modulation and the arterial modulation the red and infra-red amplitude modulation ratio percents may be chosen to represent any particular desired value of blood oxygen for each of the non-arterial and arterial waves; PA1 b. The wave shape of the non-arterial modulation and the arterial modulation may be separately specified; PA1 c. The fundamental frequency of the non-arterial modulation and the arterial modulation may be separately set; PA1 d. The amplitude of the non-arterial modulation and the arterial modulation waves may be separately set; and PA1 e. The modulations are applied as appropriate to each of the current sensed electrical analogues; red and infra-red.
Current oximetry simulator design is shown in FIG. 1. An oximeter 1 consists in part of an LED driver 2 which normally drives current to illuminate either a red or infra-red LED. The simulator 3 captures these drive currents with a current sensor 4 which creates analogs of the drive currents at point "Y".
The current sensor may be any of the current sensors in the related art, including at least:
Of these, the use of opto couplers may provide a more realistic simulation, as the LED driver is actually driving LEDs. In fact, the oximeter probe LEDs may be beneficially used as one half of such an opto coupler, permitting a full oximetry simulation including the probe. Many other current sensing schemes are known in the related art and will not be further reviewed here. Included in the sensing process is some means or method for distinguishing between the analogs of the red and infra-red oximeter drive pulses; the particular means or method being one appropriate to the LED drive current sensor(s) employed. Selection of such means or methods are well within the capabilities of a skilled practitioner, and will not be discussed further here.
The captured LED current drive analogs at point "Y" are applied to modulator 5. The pulses are modulated with the red and infra-red plethysmographic wave forms in the appropriate ratios so that the pulse amplitude modulated forms of the pulses at point "X" simulate the pulse amplitude modulation that would normally be made by the pulsing of arterial blood.
In addition to being modulated, the pulses at "X" are typically attenuated. The pulses at "X" are attenuated to simulate the bulk attenuation of non-pulsatile tissue, in addition to the modulation which simulates the variable attenuation of arterial pulsatile tissue.
The attenuated, modulated pulses at point "X" are then applied to a current driver 6 which applies the modulated pulses in the form of input current to the oximeter's photo diode amplifier 7.
The current driver may be any of the current drivers well known in the related art, including:
Of these, the opto coupler may provide a more realistic simulation, as the current driver is actually driving an isolated photo diode, which is what the oximeter expects as its current source. In fact, the oximeter probe photo diode may be beneficially used as one half of such an opto coupler, permitting a full oximetry simulation including the probe. Many other current driving schemes are known in the related art and will not be further reviewed here.
FIG. 2 shows an expanded picture of the attenuation/modulation process. The process has three inputs:
The preceding are combined by the arterial attenuation/modulation process so that the output of the process, regardless of means or method, is related to the input by: